The invention relates to the field of permanent magnets, and more particularly ferrite type magnets comprising magnetoplumbite phase.
The present invention relates to ferrite type permanent magnets based on magnetoplumbite phase according to the formula MFe12O19 where M is equal to Sr, Ba, etc., wherein the element M is partially substituted by an element R, chosen from the rare earths or bismuth, and wherein the element Fe is partially substituted by at least one transition metal T.
Such magnets are already known to show high magnetic properties, as disclosed in the Japanese application J10-149910 or in the European application EP-0 905 718 or in the international application WO99/34376.
In these applications, lanthanum La is frequently used as the element R and cobalt Co as the element T.
The manufacture of such magnets comprises the following steps:
a) formation of a mixture of raw materials either using a wet process to form a dispersion, or using a dry process to form granules,
b) roasting of the mixture at around 1250xc2x0 C. to form a clinker, comprising the desired magnetoplumbite phase, said mixture, in the form of either a dispersion or granules, being introduced into a roasting furnace,
c) wet grinding of the clinker until an aqueous dispersion of particles of particulate size of approximately 1 xcexcm is obtained, in the form of a paste containing approximately 70% dry extract,
d) the paste is concentrated and compressed in an orientation magnetic field of approximately 1 Tesla and under a pressure of 30 to 50 MPa so as to obtain an anisotropic green compact, containing 87% dry extract,
e) after drying and elimination of the residual water, sintering of the green compact,
f) final machining to obtain the magnet of predetermined shape.
The French applications No. 99 8886 and No. 99 15093 held by the applicant are also known, which disclose manufacturing methods aiming to improve certain final magnetic properties or the quality/price ratio of the magnets obtained using these methods.
According to their very varied applications, magnets must have high performances for a specific property, typically chosen from the remanence Br, generally expressed in mT, the magnetocrystalline anisotropic field Ha expressed in kA/m, the coercive field HcJ expressed in kA/m, the squareness given by hK=Hk/HcJ (%), and if applicable a performance index IP, typically taken to be equal to Br+0.5.HcJ.
This especially applies in the case of applications requiring magnets particularly showing a very high squareness and, at the same time, high values for remanence Br and the coercive field HcJ, while retaining a reasonable manufacturing cost, particularly by means of low material costs and an economic manufacturing method.
The invention relates to a method making it possible to achieve all these aims at the same time, and the magnets obtained using this method.
In the method according to the invention to manufacture ferrite type permanent magnets comprising a magnetoplumbite phase according to the formula M1-xRxFe12-yTyO19 wherein Fe and M=Ba, Sr, Ca, Pb represent the main elements, R and T being the substitute elements where R=Bi or rare earth elements, and T=Mn, Co, Ni, Zn, where x and y are typically between 0.05 and 0.5:
a) in mixing means, typically a mixer operating in batch mode, a mixture MP of the raw materials MPM, MPF, MPR and MPT relating to the elements M, Fe, R and T, respectively, is formed, typically in the form of oxide, carbonate or hydroxide powders, composed of particles P, referred to as PM, PR, PF and PT respectively, the raw material MPF relating to the element Fe, typically iron oxide Fe2O3 and the raw material MPM representing the so-called main raw materials and the raw materials MPR and MPT representing the so-called substitute raw materials MPS,
b) said mixture is roasted in a roasting furnace to form a clinker B, based on magnetoplumbite phase according to the formula M1-xRxFe12-yTyO19.
c) wet grinding of said clinker is carried out, typically in a dispersion vessel in aqueous medium, to obtain a homogeneous dispersion C of separated fine particles of average particulate size of less than 1.2 xcexcm,
d) said particles are concentrated and compressed in an orientation magnetic field to form an anisotropic, easy to handle green compact D of a predetermined shape,
e) said anisotropic green compact D is sintered to obtain a sintered element E,
f) if required, a final shaping of said sintered element E is performed, typically by machining.
This method is characterised in that, in the mixture MP in step a) of the method, at least one of the substitute raw materials MPR or MPT has a grain size GS, typically measured using the specific surface BET in m2/g and referred to specifically as GR or GT for the substitute raw materials MPR or MPT respectively, chosen according to the grain size GF of the main raw material MPF and according to the percentage by weight % S of said substitute raw material MPS with reference to said main raw material MPF given said formula of the ferrite M1-xRxFe12-yTyO19 so as to obtain a mixture MP comprising, statistically or ideally, irrespective of the formula of the ferrite, a pre-determined proportion of particles PR or PT with reference to the particles PF.
In this way, following its studies, the applicant recognised the importance of the relative grain size GR or/or GT of the substitute raw materials in question in relation both to the grain size GF of the iron oxide forming the main raw material MPF, and to the composition of the ferrite which varies with the substitution indices x and y in the ferrite formula M1-xRxFe12-yTyO19.
MPT=Co3O4 
It formulated the hypothesis that the end properties of ferrites could depend not only on the overall weight ratios between the constituents, generally taken in the divided state, but also the environment of the constituents taken at the particle scale.
By studying this field, and varying the grain size of the substitute raw materials with reference to the iron oxide, the applicant observed unexpected variations in properties, particularly in terms of the squareness given by the ratio hK=Hk/HcJ in %, Hk and HcJ being expressed in kA.mxe2x88x921, Hk being equal to H(Br-10%), i.e. the field corresponding to a magnetic induction taken to be equal to 0.9 Br and not 0.95 Br as is frequently encountered, which would have led to even higher values for the ratio hK, but would have tended to xe2x80x9ccrushxe2x80x9d the numerical values given the already high values obtained with magnets according to the state of the art.
In this way, the applicant observed significant increases in the ratio hK, all other things being equal, both in terms of the manufacturing method which is not modified significantly, and in terms of the end properties of the ferrite magnets. Indeed, as the tests demonstrate, it is remarkable to note that the method according to the invention not only makes it possible to obtain high hK ratio values but it also retains the high levels achieved for magnetic induction Br and the coercive field HcJ, which is of particular interest in practice.